Life Drain

The damage is not why you carry this. Holding it on a crowd refills your health faster than most things in the same tier can chip it away, and the more separate bodies you are touching the harder the regeneration ramps. It also speeds up how fast your natural regeneration climbs back to full, so the effect outlasts the moment you stop firing. In an invasion you can plant yourself in the middle of a wave and simply outlast it. Because none of that counts as direct healing, the Moon Lord's bite debuff cannot switch it off, where most healing options stop working.

It comes off Crimson Mimics, which means a crimson world or a trip to someone else's, and it drops often enough that a handful of mimics will produce one. Do not confuse it with a normal mimic in the caves; you want the hardmode crimson variety, which either spawns in the underground crimson or is baited out deliberately.

You point at an area rather than at a target. Everything within a square around the cursor takes damage as long as your line to that spot is unobstructed, and the range is generous enough to cover most of the screen. The red streams flowing back to you are decoration only, so nothing breaks if they clip a wall.

The drain is applied to enemies as a hidden status effect, so anything with blanket debuff immunity takes no damage over time and feeds you nothing in return. The Destroyer is the case that catches people out. Every segment of a worm counts as its own target, which sounds like the ideal scenario, but neither The Destroyer nor its probes will return a single point of health. Target dummies, on the other hand, are treated as live enemies, so you can test the whole thing at home.

The cost per use is fixed no matter how many enemies you are draining, so a wide sweep costs the same as a single target. Mana is the only real limiter, which is why a Celestial Magnet or Treasure Magnet matters here more than usual: a wider star pickup radius keeps the bar topped up and gives you effectively endless uptime in a farm or beside a star statue. Mythical is the pick for output, Celestial if you would rather push the uptime further.

The Vampire Knives heal harder, but they need the knives to actually reach a body. This does not, since a single layer of blocks between you and the target rarely stops it. Neither the healing nor the burn scales with magic damage bonuses either, so a ranger or a summoner loses nothing by keeping it in a slot and converting mana they were not using into staying alive.